VECTORS. 3-1 What is Physics? 3-2 Vectors and Scalars CHAPTER

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1 CHAPTER 3 VECTORS 3-1 What is Physics? Physics deals with a great many quantities that have both size and direction, and it needs a special mathematical language the language of vectors to describe those quantities. This language is also used in engineering, the other sciences, and even in common speech. If you have ever given directions such as Go five blocks down this street and then hang a left, you have used the language of vectors. In fact, navigation of any sort is based on vectors, but physics and engineering also need vectors in special ways to explain phenomena involving rotation and magnetic forces, which we get to in later chapters. In this chapter, we focus on the basic language of vectors. 3-2 Vectors and Scalars A particle moving along a straight line can move in only two directions. We can take its motion to be positive in one of these directions and negative in the other. For a particle moving in three dimensions, however, a plus sign or minus sign is no longer enough to indicate a direction. Instead, we must use a vector. A vector has magnitude as well as direction, and vectors follow certain (vector) rules of combination, which we examine in this chapter. A vector quantity is a quantity that has both a magnitude and a direction and thus can be represented with a vector. Some physical quantities that are vector quantities are displacement, velocity, and acceleration. You will see many more throughout this book, so learning the rules of vector combination now will help you greatly in later chapters. Not all physical quantities involve a direction. Temperature, pressure, energy, mass, and time, for example, do not point in the spatial sense. We call such quantities scalars, and we deal with them by the rules of ordinary algebra. A single value, with a sign (as in a temperature of -40 F), specifies a scalar. The simplest vector quantity is displacement, or change of position. A vector that represents a displacement is called, reasonably, a displacement vector. (Similarly, we have velocity vectors and acceleration vectors.) If a particle changes its position by moving from A to B in Fig. 3-1a, we say that it undergoes a displacement from A to B, which we represent with an arrow pointing from A to B. The arrow specifies the vector graphically. To distinguish vector symbols from other kinds of arrows in this book, we use the outline of a triangle as the arrowhead.

2 Figure 3-1 (a) All three arrows have the same magnitude and direction and thus represent the same displacement. (b) All three paths connecting the two points correspond to the same displacement vector. In Fig. 3-1a, the arrows from A to B, from A to B, and from A to B have the same magnitude and direction. Thus, they specify identical displacement vectors and represent the same change of position for the particle. A vector can be shifted without changing its value if its length and direction are not changed. The displacement vector tells us nothing about the actual path that the particle takes. In Fig. 3-1b, for example, all three paths connecting points A and B correspond to the same displacement vector, that of Fig. 3-1a. Displacement vectors represent only the overall effect of the motion, not the motion itself. 3-3 Adding Vectors Geometrically Suppose that, as in the vector diagram of Fig. 3-2a, a particle moves from A to B and then later from B to C. We can represent its overall displacement (no matter what its actual path) with two successive displacement vectors, AB and BC. The net displacement of these two displacements is a single displacement from A to C. We call AC the vector sum (or resultant) of the vectors AB and BC. This sum is not the usual algebraic sum.

3 Figure 3-2 (a) AC is the vector sum of the vectors AB and BC. (b) The same vectors relabeled. In Fig. 3-2b, we redraw the vectors of Fig. 3-2a and relabel them in the way that we shall use from now on, namely, with an arrow over an italic symbol, as in. If we want to indicate only the magnitude of the vector (a quantity that lacks a sign or direction), we shall use the italic symbol, as in a, b, and s. (You can use just a handwritten symbol.) A symbol with an overhead arrow always implies both properties of a vector, magnitude and direction. We can represent the relation among the three vectors in Fig. 3-2b with the vector equation (3-1) which says that the vector is the vector sum of vectors and. The symbol + in Eq. 3-1 and the words sum and add have different meanings for vectors than they do in the usual algebra because they involve both magnitude and direction. Figure 3-2 suggests a procedure for adding two-dimensional vectors and geometrically. (1) On paper, sketch vector to some convenient scale and at the proper angle. (2) Sketch vector to the same scale, with its tail at the head of vector, again at the proper angle. (3) The vector sum is the vector that extends from the tail of to the head of. Vector addition, defined in this way, has two important properties. First, the order of addition does not matter. Adding to gives the same result as adding to (Fig. 3-3); that is,

4 (3-2) Figure 3-3 The two vectors and can be added in either order; see Eq Second, when there are more than two vectors, we can group them in any order as we add them. Thus, if we want to add vectors,, and, we can add and first and then add their vector sum to. We can also add and first and then add that sum to. We get the same result either way, as shown in Fig That is, (3-3) Figure 3-4 The three vectors,, and can be grouped in any way as they are added; see Eq The vector - is a vector with the same magnitude as but the opposite direction (see Fig. 3-5). Adding the two vectors in Fig. 3-5 would yield

5 Figure 3-5 The vectors and - have the same magnitude and opposite directions. Thus, adding - has the effect of subtracting. We use this property to define the difference between two vectors: let = -. Then (3-4) that is, we find the difference vector by adding the vector - to the vector. Figure 3-6 shows how this is done geometrically. Figure 3-6 (a) Vectors,, and -. (b) To subtract vector from vector, add vector - to vector. As in the usual algebra, we can move a term that includes a vector symbol from one side of a vector equation to the other, but we must change its sign. For example, if we are given Eq. 3-4 and need to solve for, we can rearrange the equation as

6 Remember that, although we have used displacement vectors here, the rules for addition and subtraction hold for vectors of all kinds, whether they represent velocities, accelerations, or any other vector quantity. However, we can add only vectors of the same kind. For example, we can add two displacements, or two velocities, but adding a displacement and a velocity makes no sense. In the arithmetic of scalars, that would be like trying to add 21 s and 12 m. CHECKPOINT 1 The magnitudes of displacements and are 3 m and 4 m, respectively, and = +. Considering various orientations of and, what is (a) the maximum possible magnitude for and (b) the minimum possible magnitude? Answer: (a) 7 m ( and are in same direction); (b) 1 m ( and are in opposite directions) Sample Problem Adding vectors in a drawing, orienteering In an orienteering class, you have the goal of moving as far (straight-line distance) from base camp as possible by making three straight-line moves. You may use the following displacements in any order: (a), 2.0 km due east (directly toward the east); (b), 2.0 km 30 north of east (at an angle of 30 toward the north from due east); (c), 1.0 km due west. Alternatively, you may substitute either - for or - for. What is the greatest distance you can be from base camp at the end of the third displacement? Reasoning: Using a convenient scale, we draw vectors,,, -, and - as in Fig. 3-7a. We then mentally slide the vectors over the page, connecting three of them at a time in headto-tail arrangements to find their vector sum. The tail of the first vector represents base camp. The head of the third vector represents the point at which you stop. The vector sum extends from the tail of the first vector to the head of the third vector. Its magnitude d is your distance from base camp.

7 Figure 3-7 (a) Displacement vectors; three are to be used. (b) Your distance from base camp is greatest if you undergo displacements,, and -, in any order. We find that distance d is greatest for a head-to-tail arrangement of vectors,, and -. They can be in any order, because their vector sum is the same for any order. The order shown in Fig. 3-7b is for the vector sum Using the scale given in Fig. 3-7a, we measure the length d of this vector sum, finding (Answer) 3-4 Components of Vectors Adding vectors geometrically can be tedious. A neater and easier technique involves algebra but requires that the vectors be placed on a rectangular coordinate system. The x and y axes are usually drawn in the plane of the page, as shown in Fig. 3-8a. The z axis comes directly out of the page at the origin; we ignore it for now and deal only with two-dimensional vectors.

8 Figure 3-8 (a) The components ax and a y of vector. (b) The components are unchanged if the vector is shifted, as long as the magnitude and orientation are maintained. (c) The components form the legs of a right triangle whose hypotenuse is the magnitude of the vector. A component of a vector is the projection of the vector on an axis. In Fig. 3-8a, for example, a x is the component of vector on (or along) the x axis and a y is the component along the y axis. To find the projection of a vector along an axis, we draw perpendicular lines from the two ends of the vector to the axis, as shown. The projection of a vector on an x axis is its x component, and similarly the projection on the y axis is the y component. The process of finding the components of a vector is called resolving the vector. A component of a vector has the same direction (along an axis) as the vector. In Fig. 3-8, a x and a y are both positive because extends in the positive direction of both axes. (Note the small arrowheads on the components, to indicate their direction.) If we were to reverse vector, then both components would be negative and their arrowheads would point toward negative x and y. Resolving vector Fig. 3-9 yields a positive component b x and a negative component b y. in

9 Figure 3-9 The component of on the x axis is positive, and that on the y axis is negative. In general, a vector has three components, although for the case of Fig. 3-8a the component along the z axis is zero. As Figs. 3-8a and b show, if you shift a vector without changing its direction, its components do not change. We can find the components of in Fig. 3-8a geometrically from the right triangle there: (3-5) where θ is the angle that the vector makes with the positive direction of the x axis, and a is the magnitude of. Figure 3-8c shows that and its x and y components form a right triangle. It also shows how we can reconstruct a vector from its components: we arrange those components head to tail. Then we complete a right triangle with the vector forming the hypotenuse, from the tail of one component to the head of the other component. Once a vector has been resolved into its components along a set of axes, the components themselves can be used in place of the vector. For example, in Fig. 3-8a is given (completely determined) by a and θ. It can also be given by its components a x and a y. Both pairs of values contain the same information. If we know a vector in component notation (a x and a y ) and want it in magnitude-angle notation (a and θ), we can use the equations (3-6) to transform it. In the more general three-dimensional case, we need a magnitude and two angles (say, a, θ, and ) or three components (a x, a y, and a z ) to specify a vector.

10 CHECKPOINT 2 In the figure, which of the indicated methods for combining the x and y components of vector are proper to determine that vector? (a) (b) (c) (d) (e) (f)

11 Answer: c, d, f (components must be head to tail; must extend from tail of one component to head of the other) Sample Problem Finding components, airplane flight A small airplane leaves an airport on an overcast day and is later sighted 215 km away, in a direction making an angle of 22 east of due north. How far east and north is the airplane from the airport when sighted? KEY IDEA We are given the magnitude (215 km) and the angle (22 east of due north) of a vector and need to find the components of the vector. Calculations: We draw an xy coordinate system with the positive direction of x due east and that of y due north (Fig. 3-10). For convenience, the origin is placed at the airport. The airplane's displacement points from the origin to where the airplane is sighted. Figure 3-10 A plane takes off from an airport at the origin and is later sighted at P. To find the components of, we use Eq. 3-5 with θ = 68 (= ):

12 (Answer) (Answer) Thus, the airplane is 81 km east and km north of the airport. Problem-Solving Tactics Angles, trig functions, and inverse trig functions Tactic 1: Angles Degrees and Radians Angles that are measured relative to the positive direction of the x axis are positive if they are measured in the counterclockwise direction and negative if measured clockwise. For example, 210 and -150 are the same angle. Angles may be measured in degrees or radians (rad). To relate the two measures, recall that a full circle is 360 and 2π rad. To convert, say, 40 to radians, write Tactic 2: Trig Functions You need to know the definitions of the common trigonometric functions sine, cosine, and tangent because they are part of the language of science and engineering. They are given in Fig in a form that does not depend on how the triangle is labeled. Figure 3-11 A triangle used to define the trigonometric functions. See also Appendix E. You should also be able to sketch how the trig functions vary with angle, as in Fig. 3-12, in order to be able to judge whether a calculator result is reasonable. Even knowing the signs of the functions in the various quadrants can be of help.

13 Figure 3-12 Three useful curves to remember. A calculator's range of operation for taking inverse trig functions is indicated by the darker portions of the colored curves. Tactic 3: Inverse Trig Functions When the inverse trig functions sin -1, cos -1, and tan -1 are taken on a calculator, you must consider the reasonableness of the answer you get, because there is usually another possible answer that the calculator does not give. The range of operation for a calculator in taking each inverse trig function is indicated in Fig As an example, sin has associated angles of 30 (which is displayed by the calculator, since 30 falls within its range of operation) and 150. To see both values, draw a horizontal line through 0.5 in Fig. 3-12a and note where it cuts the sine curve. How do you

14 distinguish a correct answer? It is the one that seems more reasonable for the given situation. Tactic 4: Measuring Vector Angles The equations for cos θ and sin θ in Eq. 3-5 and for tan θ in Eq. 3-6 are valid only if the angle is measured from the positive direction of the x axis. If it is measured relative to some other direction, then the trig functions in Eq. 3-5 may have to be interchanged and the ratio in Eq. 3-6 may have to be inverted. A safer method is to convert the angle to one measured from the positive direction of the x axis. 3-5 Unit Vectors A unit vector is a vector that has a magnitude of exactly 1 and points in a particular direction. It lacks both dimension and unit. Its sole purpose is to point that is, to specify a direction. The unit vectors in the positive directions of the x, y, and z axes are labeled,, and, where the hat is used instead of an overhead arrow as for other vectors (Fig. 3-13). The arrangement of axes in Fig is said to be a right-handed coordinate system. The system remains right-handed if it is rotated rigidly. We use such coordinate systems exclusively in this book. Figure 3-13 Unit vectors,, and define the directions of a right-handed coordinate system. Unit vectors are very useful for expressing other vectors; for example, we can express and of Figs. 3-8 and 3-9 as and (3-7) (3-8) These two equations are illustrated in Fig The quantities a x and a y are vectors, called the vector components of. The quantities a x and a y are scalars, called the scalar components of (or, as before, simply its components).

15 Figure 3-14 (a) The vector components of vector. (b) The vector components of vector. 3-6 Adding Vectors by Components Using a sketch, we can add vectors geometrically. On a vector-capable calculator, we can add them directly on the screen. A third way to add vectors is to combine their components axis by axis, which is the way we examine here. To start, consider the statement which says that the vector is the same as the vector ( + ). Thus, each component of must be the same as the corresponding component of ( + ): (3-9) (3-10)

16 (3-11) (3-12) In other words, two vectors must be equal if their corresponding components are equal. Equations 3-9 to 3-12 tell us that to add vectors and, we must (1) resolve the vectors into their scalar components; (2) combine these scalar components, axis by axis, to get the components of the sum ; and (3) combine the components of to get itself. We have a choice in step 3. We can express in unit-vector notation or in magnitude-angle notation. This procedure for adding vectors by components also applies to vector subtractions. Recall that a subtraction such as = - can be rewritten as an addition = + (- ). To subtract, we add and - by components, to get where (3-13) CHECKPOINT 3 (a) In the figure here, what are the signs of the x components of and? (b) What are the signs of the y components of and? (c) What are the signs of the x and y components of +? Answer: (a) +, +; (b) +, -; (c) +, + (draw vector from tail of to head of ) Sample Problem

17 Adding vectors, unit-vector components Figure 3-15a shows the following three vectors: and Figure 3-15 Vector is the vector sum of the other three vectors. What is their vector sum KEY IDEA which is also shown? We can add the three vectors by components, axis by axis, and then combine the components to write the vector sum.

18 Calculations: For the x axis, we add the x components of,, and, to get the x component of the vector sum : Similarly, for the y axis, We then combine these components of to write the vector in unit-vector notation: (Answer) where (2.6 m) is the vector component of along the x axis and -(2.3 m) is that along the y axis. Figure 3-15b shows one way to arrange these vector components to form. (Can you sketch the other way?) We can also answer the question by giving the magnitude and an angle for 3-6, the magnitude is and the angle (measured from the +x direction) is. From Eq. (Answer) (Answer) where the minus sign means clockwise. Sample Problem Adding vectors by components, desert ant The desert ant Cataglyphis fortis lives in the plains of the Sahara desert. When one of the ants forages for food, it travels from its home nest along a haphazard search path, over flat, featureless sand that contains no landmarks. Yet, when the ant decides to return home, it turns and then runs directly home. According to experiments, the ant keeps track of its movements along a mental coordinate system. When it wants to return to its home nest, it effectively sums its displacements along the axes of the system to calculate a vector that points directly home. As an example of the calculation, let's consider an ant making five runs of 6.0 cm each on an xy coordinate system, in the directions shown in Fig. 3-16a starting from home. At the end of the fifth run, what are the magnitude and angle of the

19 ant's net displacement vector and what are those of the homeward vector that extends from the ant's final position back to home? In a real situation, such vector calculations might involve thousands of such runs. Figure 3-16 (a) A search path of five runs. (b) The x and y components of. KEY IDEA S (c) Vector points the way to the home nest. (1) To find the net displacement we need to sum the five individual displacement vectors: (2) We evaluate this sum for the x components alone, and for the y components alone, (3-14)

20 (3-15) (3) We construct from its x and y components. Calculations: To evaluate Eq. 3-14, we apply the x part of Eq. 3-5 to each run: Equation 3-14 then gives us Similarly, we evaluate the individual y components of the five runs using the y part of Eq The results are shown in Table 3-1. Substituting the results into Eq then gives us Table 3-1 Run d x (cm) d y (cm) net Vector and its x and y components are shown in Fig. 3-16b. To find the magnitude and angle of from its components, we use Eq The magnitude is To find the angle (measured from the positive direction of x), we take an inverse tangent:

21 (3-16) Caution: Taking an inverse tangent on a calculator may not give the correct answer. The answer indicates that the direction of is in the fourth quadrant of our xy coordinate system. However, when we construct the vector from its components (Fig. 3-16b), we see that the direction of is in the second quadrant. Thus, we must fix the calculator's answer by adding 180 : (3-17) Thus, the ant's displacement has magnitude and angle (Answer) Vector directed from the ant to its home has the same magnitude as but the opposite direction (Fig. 3-16c). We already have the angle ( ) for the direction opposite. Thus, has magnitude and angle (Answer) A desert ant traveling more than 500 m from its home will actually make thousands of individual runs. Yet, it somehow knows how to calculate (without studying this chapter). 3-7 Vectors and the Laws of Physics So far, in every figure that includes a coordinate system, the x and y axes are parallel to the edges of the book page. Thus, when a vector is included, its components a x and a y are also parallel to the edges (as in Fig. 3-17a). The only reason for that orientation of the axes is that it looks proper ; there is no deeper reason. We could, instead, rotate the axes (but not the vector ) through an angle as in Fig. 3-17b, in which case the components would have new values, call them and. Since there are an infinite number of choices of, there are an infinite number of different pairs of components for.

22 Figure 3-17 (a) The vector and its components. (b) The same vector, with the axes of the coordinate system rotated through an angle. Which then is the right pair of components? The answer is that they are all equally valid because each pair (with its axes) just gives us a different way of describing the same vector same magnitude and direction for the vector. In Fig we have and ; all produce the (3-18) (3-19) The point is that we have great freedom in choosing a coordinate system, because the relations among vectors do not depend on the location of the origin or on the orientation of the axes. This is also true of the relations of physics; they are all independent of the choice of coordinate system. Add to that the simplicity and richness of the language of vectors and you can see why the laws of physics are almost always presented in that language: one equation, like Eq. 3-9, can represent three (or even more) relations, like Eqs. 3-10, 3-11, and Multiplying Vectors * There are three ways in which vectors can be multiplied, but none is exactly like the usual algebraic multiplication. As you read this section, keep in mind that a vector-capable calculator will help you multiply vectors only if you understand the basic rules of that multiplication. Multiplying a Vector by a Scalar If we multiply a vector by a scalar s, we get a new vector. Its magnitude is the product of the

23 magnitude of and the absolute value of s. Its direction is the direction of if s is positive but the opposite direction if s is negative. To divide by s, we multiply by 1/s. Multiplying a Vector by a Vector There are two ways to multiply a vector by a vector: one way produces a scalar (called the scalar product), and the other produces a new vector (called the vector product). (Students commonly confuse the two ways.) The Scalar Product The scalar product of the vectors and in Fig. 3-18a is written as and defined to be (3-20) where a is the magnitude of, b is the magnitude of, and is the angle between and (or, more properly, between the directions of and ). There are actually two such angles: and Either can be used in Eq. 3-20, because their cosines are the same. Figure 3-18 (a) Two vectors and, with an angle between them. (b) Each vector has a component along the direction of the other vector.

24 Note that there are only scalars on the right side of Eq (including the value of cos ). Thus on the left side represents a scalar quantity. Because of the notation, is also known as the dot product and is spoken as a dot b. A dot product can be regarded as the product of two quantities: (1) the magnitude of one of the vectors and (2) the scalar component of the second vector along the direction of the first vector. For example, in Fig. 3-18b, has a scalar component a cos along the direction of ; note that a perpendicular dropped from the head of onto determines that component. Similarly, has a scalar component b cos along the direction of. If the angle between two vectors is 0, the component of one vector along the other is maximum, and so also is the dot product of the vectors. If, instead, is 90, the component of one vector along the other is zero, and so is the dot product. Equation 3-20 can be rewritten as follows to emphasize the components: (3-21) The commutative law applies to a scalar product, so we can write When two vectors are in unit-vector notation, we write their dot product as which we can expand according to the distributive law: Each vector component of the first vector is to be dotted with each vector component of the second vector. By doing so, we can show that (3-22) (3-23) CHECKPOINT 4 Vectors and have magnitudes of 3 units and 4 units, respectively. What is the angle between the directions of and if equals (a) zero, (b) 12 units, and (c) -12 units? Answer: (a) 90 ; (b) 0 (vectors are parallel same direction); (c) 180 (vectors are antiparallel opposite directions)

25 Sample Problem Angle between two vectors using dot products What is the angle between and? (Caution: Although many of the following steps can be bypassed with a vector-capable calculator, you will learn more about scalar products if, at least here, you use these steps.) KEY IDEA The angle between the directions of two vectors is included in the definition of their scalar product (Eq ): (3-24) Calculations: In Eq. 3-24, a is the magnitude of, or (3-25) and b is the magnitude of, or (3-26) We can separately evaluate the left side of Eq by writing the vectors in unit-vector notation and using the distributive law: We next apply Eq to each term in this last expression. The angle between the unit vectors in the first term ( and ) is 0, and in the other terms it is 90. We then have Substituting this result and the results of Eqs and 3-26 into Eq yields so

26 (Answer) The Vector Product The vector product of and, written, produces a third vector whose magnitude is (3-27) where is the smaller of the two angles between and. (You must use the smaller of the two angles between the vectors because sin and sin(360 - ) differ in algebraic sign.) Because of the notation, is also known as the cross product, and in speech it is a cross b. If and are parallel or antiparallel,. The magnitude of which can be written as is maximum when and are perpendicular to each other. The direction of is perpendicular to the plane that contains and. Figure 3-19a shows how to determine the direction of = with what is known as a right-hand rule. Place the vectors and tail to tail without altering their orientations, and imagine a line that is perpendicular to their plane where they meet. Pretend to place your right hand around that line in such a way that your fingers would sweep into through the smaller angle between them. Your outstretched thumb points in the direction of. Right-hand Rule for Cross Products

27 Figure 3-19 Illustration of the right-hand rule for vector products. (a) Sweep vector into vector with the fingers of your right hand. Your outstretched thumb shows the direction of vector =. (b) Showing that is the reverse of. The order of the vector multiplication is important. In Fig. 3-19b, we are determining the direction of so the fingers are placed to sweep into through the smaller angle. The thumb ends up in the opposite direction from previously, and so it must be that ; that is (3-28) In other words, the commutative law does not apply to a vector product.

28 In unit-vector notation, we write which can be expanded according to the distributive law; that is, each component of the first vector is to be crossed with each component of the second vector. The cross products of unit vectors are given in Appendix E (see Products of Vectors ). For example, in the expansion of Eq. 3-29, we have (3-29) because the two unit vectors and are parallel and thus have a zero cross product. Similarly, we have In the last step we used Eq to evaluate the magnitude of as unity. (These vectors and each have a magnitude of unity, and the angle between them is 90.) Also, we used the right-hand rule to get the direction of being in the positive direction of the z axis (thus in the direction of ). Continuing to expand Eq. 3-29, you can show that (3-30) A determinant (Appendix E) or a vector-capable calculator can also be used. To check whether any xyz coordinate system is a right-handed coordinate system, use the right-hand rule for the cross product = with that system. If your fingers sweep (positive direction of x) into (positive direction of y) with the outstretched thumb pointing in the positive direction of z (not the negative direction), then the system is right-handed. CHECKPOINT 5 Vectors and have magnitudes of 3 units and 4 units, respectively. What is the angle between the directions of and if the magnitude of the vector product is (a) zero and (b) 12 units? Answer: (a) 0 or 180 ; (b) 90

29 Sample Problem Cross product, right-hand rule In Fig. 3-20, vector lies in the xy plane, has a magnitude of 18 units and points in a direction 250 from the positive direction of the x axis. Also, vector has a magnitude of 12 units and points in the positive direction of the z axis. What is the vector product =? KEY IDEA Figure 3-20 Vector and. (in the xy plane) is the vector (or cross) product of vectors When we have two vectors in magnitude-angle notation, we find the magnitude of their cross product with Eq and the direction of their cross product with the right-hand rule of Fig Calculations: For the magnitude we write (Answer) To determine the direction in Fig. 3-20, imagine placing the fingers of your right hand around a line perpendicular to the plane of and (the line on which is shown) such that your fingers sweep into. Your outstretched thumb then gives the direction of. Thus, as shown in the figure, lies in the xy plane. Because its direction is perpendicular to the direction of (a cross product always gives a perpendicular vector), it is at an angle of (Answer) from the positive direction of the x axis.

30 Sample Problem Cross product, unit-vector notation If and what is? KEY IDEA When two vectors are in unit-vector notation, we can find their cross product by using the distributive law. Calculations: Here we write We next evaluate each term with Eq. 3-27, finding the direction with the right-hand rule. For the first term here, the angle between the two vectors being crossed is 0. For the other terms, is 90. We find (Answer) This vector is perpendicular to both and, a fact you can check by showing that = 0 and = 0; that is, there is no component of along the direction of either or. Copyright 2011 John Wiley & Sons, Inc. All rights reserved...